The present disclosure relates to a subscriber station for a bus system and a method for time-optimized data transmission in a bus system.
The CAN bus system has become widely used for communication between sensors and control devices such as, for example, in motor vehicles. In the CAN bus system, messages are transmitted by means of the CAN protocol as is described in ISO11898. Automobile bus systems, in particular, are developing continuously to increased bandwidths, lower periods of latency and stricter real-time capability. In recent times, techniques have also been proposed for this such as, for example, CAN FD in which the maximum possible data rate is increased beyond a value of 1 MBit/s by using a higher clock rate in the area of the datasheets. Such messages will also be called CAN FD frames or CAN FD messages in the text which follows. In the case of CAN FD, the useful data length is expanded from 8 up to 84 bytes and the data transmission rates are distinctly higher than in the case of CAN. This is also specified in the current ISO-CD-11898-1 as CAN protocol specification with CAN FD.
Apart from primarily functional additions such as, e.g. TTCAN, the extension of the CAN protocol or CAN standard has been extended in recent times with CAN FD, particularly with regard to the possible (higher) data rate and the usable data packet size, retaining the original CAN characteristics, in particular in the form of the arbitration. In the arbitration, it is determined which of the subscriber stations of the bus system, as a transmitter, has at least temporarily an exclusive, collision-free access to a bus line of the bus system.
On a CAN bus, each message begins with a dominant SOF bit (SOF=Start of Frame). This is followed by an arbitration field with 11 bits followed by control bits and an optional extended arbitration field with 29 bits.
In the arbitration field of the CAN, the logical states of individual bits are mapped dominantly and recessively by the electrical states. According to the present state of the art, the bits on the CAN bus are coded in this manner for the entire frame or the entire message. This method needs 12 bit times for a standard identifier and 32 bit times for an extended identifier.
According to the Classic CAN specification and also according to the new specification according to ISO-CD-11898-1, which also includes CAN FD messages, the CAN bit arbitration ends at the bit which comes after the last identifier bit, excluding stuff bits. Up to this bit, a transmitter of a classic CAN or CAN FD message, which transmits a recessive bit but, instead, sees a dominant bit on the CAN bus, considers the arbitration as lost and becomes the receiver of a CAN message.
In the increase of the CAN transmission rate, the arbitration field limits the netto data rate. This is because the prioritizing and non-destructive arbitration method on the CAN bus requires that each bit is propagated to each subscriber station or bus node and back.
With a given physical extent of the bus system or network, the Baud rate can thus not be increased arbitrarily during the arbitration.
To increase the CAN transmission rate further, it would be possible to dispense with the arbitration. However, the arbitration characteristics, being prioritized and non-destructive, are features of the CAN protocol which make it unique and are greatly appreciated by the users.